Stress concentration degree affects spontaneous magnetic signals of ferromagnetic steel under dynamic tension load

Under the excitation of geomagnetic field, stress can induce spontaneous magnetic signals in ferromagnetic materials, which can be potentially applied to estimate the damage degree. In this research, the normal component of magnetic field, H_p(y), was measured during dynamic tension test on the surfaces of ferromagnetic specimens having different stress concentration factors, α constant amplitude sinusoidal tensile load cycles were applied on 18CrNi4A steel plate specimens whose α were respectively 1, 3 and 5. Then correlation of H_p(y) values’ changes with loading cycles reported. The results indicate that stress concentration factor, α extremely affects the magnetic signal, the absolute value of the gradient, |K|, increases with the increase of α. Meanwhile, |K| also goes up when loading cycle increases in the initial state. However, after a few cycles, those H_p(y) curves corresponding to different cycles become similar before cracking.     

Stress dependence of the spontaneous stray field signals of ferromagnetic steel

Measuring spontaneous stray field signals provides a promising tool to analyze the stress in ferromagnetic materials. However, strong initial stray field signals on the surface of ferromagnetic materials originating from various manufacturing processes can disturb stress-induced stray field signals. Consequently, it is necessary to conduct a study that will clarify the stress dependence of spontaneous stray field signals by eliminating initial random signals. In the present work, the focus is placed on sheet specimens that have a clean initial magnetic state by means of vacuum heat treatment. Measurements of the normal component H_p(y) signals of stray field were performed during the whole tensile test. The results showed that the stress-induced H_p(y) signal curve had good linearity after loading, i.e., the slope coefficient K_s increased continuously in the elastic deformation stage but decreased slightly during the plastic deformation stage. This phenomenon was discussed and explained from both the stress-induced effective magnetic field and residual compressive stress viewpoints.     

Residual magnetic field variation induced by applied magnetic field and cyclic tensile stress

Magnetic memory testing (MMT) method is a novel non-destructive testing technique due to its unique advantages of stress concentration identification and early damage detection for ferromagnetic materials. However, a thorough understanding of the impact of exciting magnetic source and cyclic stress on the residual magnetic field variation has not been clearly addressed. The surface magnetic memory signal H_p(y) induced by applied magnetic field and cyclic tensile stress was measured throughout the fatigue process. The correlation of H_p(y) and its gradient K changes with loading cycles and applied magnetic field intensity H reported. The results show that applied magnetic field can only change the magnitude of MMT signal instead of changing the H_p(y) curve׳s profile. The H_p(y) value increases with the increase of the H, and the K value is approximately linear to the H. The maximum gradient K_max indicating the degree of stress concentration increases with the increase of either stress cycles or H. The phenomenon was also discussed from the view of the magnetic dipole in a ferromagnet.     

铁磁性激光熔覆层应力的金属磁记忆评价

基于压磁理论,探讨激光熔覆层应力的磁记忆评价机理.基于激光熔覆层SEM观察对磁记忆法向分量H_p(y)信号进行分析.结果表明,随应力增大,激光熔覆层磁记忆法向分量H_p(y)曲线以过零点为中心呈逆时针转动,磁畴由无序向有序转变使得H_p(y)曲线斜率逐渐变大,当应力达到520 MPa(小于试样屈服强度)时,再随应力的增大,H_p(y)曲线斜率逐渐变小.分析认为,激光熔覆层的快速凝固各向异性组织及其中界面导致激光熔覆层呈不均匀塑性变形过程是引起上述结果的主要原因,也是应力小于试样屈服极限时,H_p(y)曲线斜率随应力增大呈减小趋势变化的原因之一.     

Magnetoresistance of the La<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> single crystal

The dependence of the resistance ρ of the La_0.7Ca_0.3MnO₃ single crystal on the temperature (in a range of 77 < T < 410 K) and magnetic field H is studied. The dependence of the magnetoresistance Δρ/ρ of the ferromagnetic phase on the field is shown to be determined by the competition of two mechanisms. In low magnetic fields, the magnetoresistance is positive Δρ/ρ > 0 and is determined by changes in the resistance with changing magnetization orientation with respect to the crystallographic axes; in high magnetic fields, the magnetoresistance is negative Δρ/ρ < 0, since it is the suppression of spin fluctuations in the magnetic field that plays the principal role. The phase transition from the ferromagnetic to paramagnetic state is a first-order transition close to the second-order one. In the transition range, the magnetoresistance is determined by the resistivity in the zero field ρ(T) and by the shift of the transition temperature T _C(H) in the magnetic field. In the paramagnetic state, the resistivity ρ(T) has an activation character; similarly to the magnetoresistance of other lanthanum manganites, the magnetoresistance of this single crystal is controlled by a change in the activation energy in the magnetic field.     

Specific heat and magnetocaloric effect of the Mn5Ge3 ferromagnet

The specific heat C_p(T) of the Mn₅Ge₃ ferromagnet has been studied in a wide temperature range of 2–400 K. The ferromagnetic ordering has been confirmed at T_C = 297.4 K, however the behaviour of C_p(T,H) in small magnetic fields suggests a presence of a weak antiparallel component in the magnetic structure. The combination of the C_p(T,H) and magnetization M(T,H) measurements enabled the determination of not only the isothermal magnetic entropy change ΔS_M but the adiabatic temperature change ΔT_ad as well. It has been also found that the mechanical milling process leads only to a moderate drop of the magnetocaloric effect. The reduction of the grain size by 50% decreases the relative cooling power by 28%.     

Investigation of the dependency of the upper critical magnetic field on the content x in Y1−xYbx/2Gdx/2Ba2Cu3O7−y superconducting structures

The Y_1−xYb_x/2Gd_x/2Ba₂Cu₃O_7−y superconducting samples for x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 were prepared by using the solid-state reaction technique. Resistivity measurements of the samples were performed in QD–PPMS system under different magnetic fields up to 5 T in zero fields cooling regime. Using the resistivity data, the upper critical magnetic field H_c2(0) at T = 0 K for 50% of R_n was calculated by the extrapolation H_c2(T) to the temperature T = 0 K. The coherence length in T = 0 K were calculated from H_c2(0) and the effects of x in the composition on both the coherence length and the upper critical magnetic field were examined. The results showed that H_c2(0) varied from 84.05 to 122.26 T with the content x. The upper critical magnetic field in the temperature T = 0 K slightly decreased with increasing the content x. Using the content x, the upper critical magnetic field can be controlled and this can be used in the superconductivity applications.     

Magnetic anisotropy of Ho–Fe–Co–Cr intermetallic compounds

Starting with a Ho₃(Fe_1−xCo_x)_29−yCr_y, (x,y) = (0.6,4.5) and (0.8,5.5) nominal stoichiometry, a disordered variant of the hexagonal 2:17 phase (Th₂Ni₁₇-type, S.G. P6₃/mmc) occurs, since both the monoclinic 3:29 and the transition-metal-rich disordered Th₂Ni₁₇-type hexagonal compounds have the same rare earth to transition metal ratio, 1:9.7. The magnetic properties and the magnetocrystalline anisotropy of these compounds have been investigated. The anisotropy constant, K's, and the anisotropy field, μ₀H_A, values have been deduced from the magnetization curves measured on powder samples magnetically aligned in a rotating magnetic field. The compound with (x,y) = (0.8,5.5) shows a compensation point at about 55 K. The magnetic anisotropy of both compounds is that of easy-plane from room temperature to low temperatures down to 5 K.     

Effect of dysprosium and cobalt on the temperature dependence of magnetization and phase composition of a material of the Nd-Dy-Fe-Co-B system

Sintered magnetically hard materials with composition (Nd_1−x Dy_x)_13–16(Fe_1−y Co_y)_resAl_0.5−1B_7–9 (x = 0.11–0.71, y = 0.19–0.34) are investigated. The role of cobalt and dysprosium in the variation of magnetic properties of the material (residual induction, coercive force) is studied. The phase composition of the material and its effect on magnetic properties are determined. Amethod for computing the induction temperature coefficient of a material on the basis of published data on the physical characteristics (the Curie temperature, the density, the molecular mass) of the main magnetic phase (Nd, Dy)₂(Fe, Co)₁₄B is presented. __________ Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 4, pp. 3–10, April, 2007.     

Molecule- and polymer-based magnets,a new frontier

Molecule- and polymer-based magnets prepared by relatively low temperature organic synthetic methodologies are of increasing interest in materials research. The three distinct models enabling the design of new molecule- and polymer-based magnetic materials are given. Bulk ferromagnetic behavior with hysteresis was first observed below the critical (Curie) temperature, T_c, of 4.8 K for [Fe^III(C₅Me₅)₂]⁺ [TCNE] ⁻ (Me = methyl; TCNE = tetracyanoethylene). Since then, many new classes of molecule- and polymer-based magnets have been discovered, including those with T_c above room temperature, such as V(TCNE)_x· y(CH₂Cl₂). The important roles of spin and spatial anisotropy and also disorder are illustrated. The, anticipated properties of such molecule- and polymer-based magnetic materials may enable their use in future generations of electronic, magnetic and/or photonic/photronic devices ranging from information storage and magnetic imaging to static and low frequency magnetic shielding and magnetic induction.     

Magnetic specific heat and magnetoresistance of URhSi

Specific heat of a single crystalline URhSi was measured by a relaxation method in a temperature range 0.3–25 K in magnetic fields up to 8 T applied along the two of the principal axes. The low-temperature specific heat exponentially decays with magnetic field. The decay is much faster in fields applied along the easy magnetization direction (the c-axis) than for the hard axis (the a-axis) case. A strong upturn in c_p/T versus T below 0.6 K that disappears with application of magnetic field is observed suggesting possible magnetic or superconducting phase transition at lower temperatures. The electrical resistivity in the vicinity of the ferromagnetic phase temperature is found to be reduced by more than 50% upon application of magnetic field of 8 T applied along the c-axis. URhSi represent an itinerant ferromagnetic system with influence of spin fluctuations.     

Intercalation of stable organic radicals into layered copper hydroxides

We report the intercalation of a stable organic radical, 3-carboxy-2,2,5,5-tetramethyl-1-pyrrolidine-1-oxyl anion (PROXYL⁻) into the layered copper hydroxides, Cu₂(OH)₃(CH₃CO₂)·H₂O, by means of the anion exchange. The powder X-ray diffractions reveal the anion exchange, showing an increase in the interlayer distance. However, the elemental analyses indicate that the obtained material is Cu₂(OH)_3.5(PROXYL)_0.5·H₂O. The temperature dependence of the magnetic susceptibilities for the intercalation compound indicates a dominant antiferromagnetic interaction in contrast to the intralayer ferromagnetic interaction in the parent compound. The EPR spectra of the intercalation compound consist of the absorption of the organic radical, whose temperature-variable follows the Curie law. The observed antiferromagnetic properties can be ascribed to a transition in the magnetic properties of the [Cu₂(OH)₃]⁺ layer from ferromagnetic to antiferromagnetic upon intercalation.     

Composite of polyaniline containing iron oxides with nanometer size

A composite of polyaniline (PANI) containing iron oxides with nanometer size was synthesized by a chemical method. The structure, electrical and magnetic properties of the resulting PANI-Fe_xO_y composites depend on the pH value of the reaction medium and temperature. For the basic preparation conditions (e.g. pH 14), the resulting PANI-Fe_xO_y, composite can be attracted by a magnet at room temperature, and its magnetization with the applied magnetic field exhibits a hysteresis loop with H_c = 0. It has been demonstrated that nanocrystalline ferromagnetic particles of Fe₃O₄ with 14.0 nm size correspond to super-paramagnetic properties observed from PANI-Fe_xy composites. For the acidic conditions (e.g. pH1), on the other hand, the electrical properties of the resulting PANI-Fe_xO_y composite show a semiconducting behavior. By comparison with PANI-Cl⁻¹, the lower conductivity (about 10⁻⁴ S/cm) for PANI-Fe_xO_y composite may be due to the presence of iron oxides in the composite, although its doping degree is the same as that of PANI-Cl⁻.     

Impact of plastic deformation on magnetoacoustic properties of Fe–2%Si alloy

The paper presents the results of a complementary study including magnetic hysteresis loops B(H), magnetic Barkhausen noise (MBN) and magnetoacoustic emission (MAE) signals measurements for plastically deformed Fe–2%Si samples. The investigated samples had been plastically deformed with plastic strain level (ε_p) up to 8%. The properties of B(H) loops are quantified using the coercivity H_C and maximum differential permeability μ_rmax as parameters. The MBN and MAE voltage signals were analysed by means of rms-like voltage (Ub and Ua, respectively) envelopes, plotted as a function of applied field strength. Integrals of the Ub and Ua voltages over half of a period of magnetization were then calculated. It has been found that H_C and integrals of Ub increase, while μ_rmax decreases monotonically with increasing ε_p. The MAE (Ua) peak voltage at first decreases, then peaks at ε_p≈1.5% and finally decreases again. The integral of the Ua voltage at first increases for low ε_p and then decreases for ε_p>1.5%. All those various dependence types suggest the possibility of detection of various stages of microstructure change. The above-mentioned results are discussed qualitatively in the paper. Some modelling of the discussed dependency is also presented.     

Composition-controlled synthesis, structure and magnetic properties of ternary FexCoyNi100−x−y alloys attached on carbon nanotubes

Fe_xCo_yNi_100−x−y alloy nanoparticles with controllable compositions attached on the surface of carbon nanotubes (CNTs) were synthesized using an easy two-step route including adsorption and reduction processes. The nanocomposites have been characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), energy-disperse X-ray spectroscopy (EDS) and vibrating sample magnetometer (VSM). The effect of the alloy composition on microstructure and magnetic properties of ternary FeCoNi alloys attached on carbon nanotubes have been studied. During the nominal composition range (x = 21, 24, 33, 37, 46 and y = 60, 46, 48, 48, 35), Fe_xCo_yNi_100−x−y alloy nanoparticles attached on CNTs are quasi-spherical, fcc–bcc dual phase, and the coercivity (H_c) and saturation magnetization (M_s) vary with the alloy composition. The H_c of Fe_xCo_yNi_100−x−y alloy nanoparticles attached on CNTs decreases and M_s increases with increasing Fe content. These demonstrate that the two-step route is promising for fabricating alloy nanoparticles attached on CNTs for magnetic storage and ultra high-density magnetic recording applications.     

Magnetic field effects in decamethyl ferrocenium tetracyanoethanide

The specific heat C(H,T) of (DMeFc) (TCNE) in the temperature range of 2–12 K in the presence of applied magnetic fields is reported. Previous results had shown anomaly at 4.82 K corresponding to a transition at T_c from a high temperature one-dimensional (1D) magnet (above T_c) to a three-dimensional (3D) macroscopic ferromagnet (below T_c). The results show a dramatic behavior of the peak associated with the ferromagnetic transition. The peak height decreases with increasing magnetic field, i.e. magnetic fields smear the transition. For small values of applied magnetic fields the peak associated with the ferromagnetic transition is completely smeared. The results are indicative of anisotropic chains with enhanced 1D coherence and frustration in interchain coupling. The spin-wave spectrum is dominated by the 1D chains below T_c.     

Synthesis and properties of polyaniline–cobalt coordination polymer

Polyaniline–cobalt coordination polymer (abbreviated as PANI-Co) was synthesized using peroxydisulphate as an oxidant in the solution containing 0.1 mol dm⁻³ aniline, 0.5 mol dm⁻³ HCl and an adequate content of CoCl₂·6H₂O at room temperature. The conductivity of PANI-Co was 0.5 S cm⁻¹. The cyclic voltammogram results indicated that the PANI-Co film was of electrochemical activity, and the EPR spectrum showed that there were unpaired electrons in the PANI-Co. The relationship between magnetization (M) and the applied magnetic field (H) suggests that PANI-Co was soft ferromagnetic material at room temperature. Thus, the PANI-Co was both conductive and ferromagnetic. Moreover, UV–vis and FTIR spectra showed that there existed a strong interaction between Co²⁺ and PANI chains.     

Giant magnetocaloric effect in a Heusler Mn50Ni40In10 unidirectional crystal

A modified high-pressure optical zone-melting technique was used to grow a Mn-rich Heusler Mn₅₀Ni₄₀In₁₀ unidirectional crystal. Experimental results showed that the produced unidirectional crystal underwent a magnetic transition in austenite, followed with a martensitic transformation from a ferromagnetic austenite to a ferromagnetic martensite upon cooling. Under a magnetic field change of 30 kOe, the total effective refrigeration capacities (RC_total) reached as high as 231.58 J/kg when the magnetic field was applied along parallel to the crystal growth direction, or 246.79 J/kg when the magnetic field was applied along perpendicular to the crystal growth direction. It was suggested that this unidirectional crystal growing technique may provide an effective approach to enhance the magnetocaloric effect of Mn-rich Heusler materials.     

Repassivation behaviour of 316L stainless steels using new type of abrading electrode technique

Abstract

In this paper, a novel electrode abrading technique is designed to improve the conventional methods. By employing the new depassivation method, 316L stainless steel has been tested in 0.05M H₂SO₄ solution with and without chloride ion. The repassivation kinetics has been analysed in terms of the current density flowing through the abraded surface bare metal surfaces. The results are described by the linear relationship of log i(t) versus log t. At potentials within the passive region, the passive film grew according to the high field ion conduction model in which log i(t) is linearly proportional to 1/q(t). The repassivation process was analysed in three stages according to different kinetics. The effect of applied potential and chloride ion concentration on repassivation kinetics was also discussed, and the change of calculated film thickness is more sensitive to applied potential than chloride ion concentration.